Mechanical vibration applied to a main body of a microphone causes vibration of a microphone unit to convert the vibration into vibratory noise signals in the microphone unit. The vibratory noise signals are mixed into original audio signals converted in the microphone unit. The mechanical vibration applied to the main body includes impactive vibration such as fall or collision, continuous vibration due to mechanical motion of an apparatus containing the microphone, and vibration due to friction on the main body.
A condenser microphone is a type of microphone. Vibratory noise signals are generated in response to vibration applied to a first-order pressure-gradient microphone having cardioid or hyper-cardioid unidirectivity, omnidirectional unidirectivity, or bidirectivity. In particular, in a narrow-directivity condenser microphone including an acoustic tube, mechanical vibration of its main body causes the mass of air in the acoustic tube to be applied to a diaphragm, and the inertia of the mass of the air moves the diaphragm to generate large vibratory noise signals. In the first-order pressure-gradient condenser microphone, the amplitudes of vibratory noise signals are significantly different depending on its directivity, increase with bidirectivity, and decrease with omnidirectivity.
In the narrow-directivity condenser microphone including the acoustic tube, its narrow-directivity decreases in a low-frequency range depending on the length of the acoustic tube. The acoustic tube is designed so as to have a practical length, to operate as a first-order pressure-gradient component in a low-frequency range in which its narrow-directivity decreases, and to have its narrow-directivity even in a low-frequency range. Vibratory noise causes vibratory force due to the mass of air moving together with the diaphragm in addition to vibration of the diaphragm of a microphone unit. A longer acoustic tube therefore increases the vibratory noise.
As described above, the vibratory noise generated in the condenser microphone depends on its directivity, which is also described in Japanese Patent No. 2520929. Japanese Patent No. 2520929, in FIG. 7, describes differences in the sensitivity of microphones to mechanical vibration among bidirective, unidirective, and omnidirectionally unidirective microphones with reference to an omnidirective microphone. As is apparent from FIG. 7 in Patent No. 2520929, the sensitivity to vibration in a middle-frequency range is the highest in the bidirective microphone and decreases in the order of the unidirective, omnidirectionally unidirective, and omnidirective microphones.
Japanese Patent No. 2520929 also describes mechanical vibration absorption for a condenser microphone. In detail, a main body or a microphone unit of the microphone is supported with viscoelastic rubber to mechanically isolate the main body from the microphone unit. Mechanical vibration absorption up to a low-frequency range by a mechanical structure however results in impractical waggly support of the microphone unit through flexible viscoelastic material.
Japanese Patent No. 2520929 therefore proposes a condenser microphone including a piezoelectric vibration pickup for detecting vibratory noise, a condenser microphone unit, means for mechanical vibration absorption including a first vibration absorption mechanism and a second vibration absorption mechanism for supporting the pickup, and means for electric cancel including an amplitude/phase characteristic corrector circuit and a level corrector circuit for correcting an amplitude and a phase of output signals of the pickup so as to equalize the amplitude and the phase to an amplitude and a phase of output signals of the condenser microphone unit and a differential circuit for receiving the output signals of the condenser microphone unit and the output signals of the level corrector circuit to output differential output signals of these output signals.
FIG. 2 illustrates an exemplary circuit of a conventional condenser microphone, which is substantially the same as a circuit of the condenser microphone described in Japanese Patent No. 2520929. In FIG. 2, audio signals electro-acoustically converted in the condenser microphone unit 7 are converted into low impedance by an impedance converter circuit 8 including an FET 9 as a main circuit element and are outputted. Vibratory noise signals detected by a piezoelectric vibration pickup 10 are converted into low impedance by an impedance converter circuit 11 including an FET 111 as a main circuit element and are outputted. The impedance-converted vibratory noise signals are adjusted to a level that corresponds to the level of the audio signals with the level corrector circuit 12 including a variable resistor 121 (for adjusting levels) and are then inputted to a buffer amplifier 13 including an FET 131 as a main circuit element. Output signals of the buffer amplifier 13 are outputted through a low-pass filter 14 including a variable resistor 141, an electrolytic capacitor 142, and a capacitor 143. The variable resistor 141 and the electrolytic capacitor 142 are connected between an input terminal and an output terminal of the low-pass filter 14 in series, and a connection node between the variable resistor 141 and the electrolytic capacitor 142 is connected to the ground GND through the capacitor 143.
Audio signals outputted from the microphone unit 7 through the impedance converter circuit 8 are outputted through a buffer 15 including a transistor 151 having emitter-follower connection as a main circuit element. Vibratory noise signals are outputted from the piezoelectric vibration pickup 10 through the impedance converter circuit 11, the level corrector circuit 12, the buffer amplifier 13 and the low-pass filter 14 and then are outputted through a buffer 16 including a transistor 161 having emitter-follower connection as a main circuit element. This circuitry is connected to an external circuit through a connector including three pins. Of the three pins, a first pin 21 is connected to the ground, a second pin 22 to the emitter of the transistor 151 in the buffer amplifier 15, and a third pin 23 to the emitter of the transistor 161 in the buffer amplifier 16. Hot and cold signals of a balanced output are outputted from the second and third pins 22 and 23, respectively.
The first, second, and third pins 21, 22, and 23 are connected to a mixer through a cable. The mixer is not shown in the drawing. The mixer includes a phantom power supply and a differential circuit for outputting differential signals of audio signals from the microphone unit 7 and vibratory noise signals from the piezoelectric vibration pickup 10. The differential circuit is disposed in a head amplifier of the mixer and receives reversed-phase signals of the vibratory noise signals as a balanced input to thereby subtract the audio signals from the vibratory noise signals. The vibratory noise generated by the vibration is canceled with the vibratory noise signals.
The input of the mixer cannot cancel the vibratory noise signals without maintain the balance between the hot and cold signals. In the example circuit shown in FIG. 2, the level corrector circuit 12 is thus used to correct the level of the vibratory noise signals so as to corresponds to the level of the audio signals. The variable resistor 141 for adjusting the characteristics of the low-pass filter 14 is used to match the low-pass filter 14 with the frequency characteristics of the vibratory noise signals to be canceled. The piezoelectric vibration pickup can generate an output having a constant level in response to constant acceleration independently of a change in a frequency. In contrast to that, the level of the vibratory noise signals generated in the microphone decreases with a higher frequency in a frequency band in which sound pressure sensitivity is flat. The vibratory noise signals detected by the piezoelectric vibration pickup 10 are processed with the low-pass filter 14 to have a similar frequency response to that of the vibratory noise signals generated with the microphone unit 7. The level of the vibratory noise signals is matched with the level of the vibratory noise signals generated with the microphone unit 7 to thereby cancel the vibratory noise signals.